1. Field of the Invention
The present invention relates to a pixel circuit and an organic light emitting display device using the same, and more particularly, to a pixel circuit capable of compensating a threshold voltage of a driving transistor for controlling an operation state of light emitting device, and a method for driving thereof and an organic light emitting display device using the same.
2. Discussion of the Related Art
According to a recent development of multimedia, there is an increasing demand for a flat panel display. In order to satisfy this increasing demand, various flat panel displays such as liquid crystal display, plasma display panel, field emission display and light emitting display are practically used. Among the various flat panel displays, the light emitting display has been attracted as a next-generation flat panel display owing to advantages of rapid response speed (response speed below 1 ms) and low power consumption. In addition, the light emitting display can emit light in itself, whereby the light emitting display does not cause a problem related with a narrow viewing angle.
Generally, the light emitting display is a display device which emits light by electrically exciting a light emitting material. According to the material and structure, the light emitting display may be classified into an inorganic light emitting display device and an organic light emitting display device.
FIG. 1 is a circuit diagram illustrating a pixel circuit of an organic light emitting display device according to the related art.
Referring to FIG. 1, the pixel circuit of the organic light emitting display device according to the related art includes a switching transistor (ST), a driving transistor (DT), a capacitor (C) and a light emitting device (OLED).
The switching transistor (ST) is switched by a scanning signal supplied to a scanning line (SL), whereby a data voltage (Vdata) supplied to a data line (DL) is supplied to the driving transistor (DT).
The driving transistor (DT) is switched by the data voltage (Vdata) supplied from the switching transistor (ST), to thereby control a data current (Ioled) flowing from a driving power source (Vdd) to the light emitting device (OLED).
The capacitor (C) is connected between gate and source terminals of the driving transistor (DT), wherein the capacitor (C) stores a voltage corresponding to the data voltage (Vdata) supplied to the gate terminal of the driving transistor (DT), and turns on the driving transistor (DT) by the stored voltage.
The light emitting device (OLED) is electrically connected between a drain terminal of the driving transistor (DT) and a ground power source (Vss), wherein the light emitting device (OLED) emits the light by the data current (Ioled) supplied from the driving transistor (DT). In this case, the data current (Ioled) flowing in the light emitting device (OLED) is determined based on a voltage (Vgs) between the gate and source terminals of the driving transistor (DT), a threshold voltage (Vth) of the driving transistor (DT), and the data voltage (Vdata).
In the pixel circuit of the organic light emitting display device according to the related art, a level of the data current (Ioled) flowing from the driving power source (Vdd) to the light emitting device (OLED) is controlled by switching the driving transistor (DT) according to the data voltage (Vdata) so that the light emitting device (OLED) emits the light, to thereby display a predetermined image.
However, in case of the pixel circuit of the organic light emitting display device according to the related art, the data current (Ioled) flowing in the light emitting device (OLED) may be changed by a deviation of the threshold voltage (Vth) of the driving transistor (DT) and a voltage drop of the driving power source (Vdd). Thus, even though the data voltage (Vdata) is identically applied to the pixel circuit of the organic light emitting display device according to the related art, the data current (Ioled) output from each driving transistor (DT) is changed so that it is difficult to realize uniformity in a picture quality.
In addition, according as the organic light emitting display device is increased in size, the above problems related with the deviation of threshold voltage (Vth) of the driving transistor (DT) and the voltage drop of the driving power source (Vdd) become more serious, whereby the picture quality is deteriorated in the large-sized organic light emitting display device.